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4-Wire
Measurement
Like a test bench in your PC, data acquisition cards can now handle multiquadrant operation and four-wire measurement, offering versatile test options that ensure that you get accurate readings.
Jon Semancik, Keithley Instruments
Once found only in benchtop measurement instruments, 4-wire sourcing and sensing and multiquadrant operation are now available on PC-based data acquisition (DA) cards. By using one pair of leads to supply test current to the device under test (DUT) and a separate pair of leads to make a voltage measurement, the 4-wire method eliminates the measurement leads’ resistance as a source of error and improves accuracy. With the addition of four-quadrant operation, PC-based cards can now sink or source voltage and current in any combination. This flexibility gives you a wider range of test options, which in turn simplifies test setup and reduces equipment requirements.
4-Wire Remote Sensing
The 4-wire remote sensing technique connects a second pair of leads directly to the DUT so that the sourcing instrument can also measure (sense) the actual voltage present at the DUT. Ideally, these measurements are made by a high-input impedance measurement subsystem that is tightly integrated in the sourcing instrument. The source can then adjust its analog output automatically and transparently to ensure that the specified voltage or current is applied to the DUT, regardless of cable lengths and interconnection losses.
This capability is important when sourcing and measuring signals over long distances. When using only a single pair of leads (the two-wire method), significant voltage drops can occur as the result of resistance in the feed cables, device interconnections, and terminations, meaning that the voltage delivered to the DUT at the end of the cable might not be the programmed value. Similarly, measured signals could be lower than the actual level at the DUT.
Experimental Testing
Figure 1. Conventional 2-wire analog output connections assume that the value programmed for analog output voltage or current will be delivered to the device under test (DUT). This assumption requires that the resistance of intervening cabling and terminations be 0  -an unlikely scenario in the real world.
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To prove the effectiveness of the 4-wire method over the 2-wire approach, Keithley Instruments programmed a known voltage from a 4-wire-capable analog source board and applied it to the DUT using cables of several different lengths. It programmed the outïut voltage with ExceLINX software, and the resistance of the cable was measured using a multimeter/DA system set to 4-wire mode. For comparison purposes, the voltage at the DUT was also measured in the 2-wire mode without remote sense.
Figure 2. In analog output with 4-wire remote sensing, the instrumentation simultaneously sources voltage or current and senses the voltage drop across the device under test (DUT). You can use this value to adjust the source's output and achieve the desired excitation to the DUT.
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Tests were performed first with the load connected to the analog output in an ordinary 2-wire source mode (see Figure 1). Then the load was connected in a 4-wire source/measure mode (see Figure 2).
The data in Table 1 indicate that cabling and device interconnections can be significant sources of error.
In all tests, 8 VDC was programmed from the analog output source. Results with the 4-wire source mode showed that 8 VDC was delivered to the DUT. With the 2-wire mode, the resistance of the leads, which ranged from 0.5 to 4 , had a discernible effect on the DUT voltage.
TABLE 1
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Sourcing Error vs. Cable Length
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Signal Path (Per Lead)
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Resistance
( per lead)
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Programmed
Value (V)
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2-Wire Mode
Measured Value
(Volts w/o Sense)
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4-Wire Mode
Measured Value
(Volts w/Sense)
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10 ft. cable
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0.5266
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8.000
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7.999
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8.000
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100 ft. cable
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2.5
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8.000
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7.920
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8.000
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6 relays,
4 interconnections,
20 ft. cable
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4.06
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8.000
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7.360
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8.000
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Multiquadrant Operation
Figure 3. Think of source/sink analog output operation as occurring in four quadrants. In each quadrant, voltage is either + or -, and current is driven into the device under test (source) or absorbed from it (sink).
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The recent addition of four-quadrant operation to PC-based DA cards has its roots in benchtop systems. Like their high-end counterparts, these cards can operate in all four voltage/current quadrants (Figure 3), sinking or sourcing any combination of current and voltage. Ideally, the source should be capable of robust voltage and current sinking or sourcing without external excitation.
Two examples—a resistive load test and a battery charge and discharge test—illustrate the process of configuring a constant current supply to function in quadrants 1 and 3. The key to generating the desired output current is to use the proper shunt resistor. Place the resistor in series with the load, and maintain the analog output across the shunt at a level that will drive the desired current through the load/shunt pair.
Figure 4. The constant current configuration operates on the principle that the voltage drop across RSHUNT can be measured and used to adjust the source's output to maintain a specific current through the circuit, even though RLOAD may vary.
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Quadrant 1: Resistive Load Test
For a single-quadrant test, the test sample (the resistive load) requires a constant current to be applied and controlled, independent of the load. From the load resistance and current requirements, you can derive the proper shunt resistor value using Ohm’s law and the sum of the series resistance of the load/shunt combination (see Figure 4).
Assume that the load resistance is 490 , the current that you want to pass through the circuit is 20 mA, and the applied voltage is 10 V. From Ohm’s law, you can see:
| Vao | = (Rtotal × I) |
| = (Rshunt + Rload ) × 20 mA |
| 10 V | = (Rshunt + 490 ) × 20 mA |
Therefore:
| Rshunt + 490 | = 10 V / 20 mA IV |
| Rshunt + 490 | = 500 |
| Rshunt | = 500 W - 490 W = 10 |
| (1) |
The analog output voltage will now be based on the voltage sensed across the shunt resistor. You can determine the acceptable shunt voltage levels using Ohm’s law and the specified current:
| Vmaxshunt | = 20 mA × Rshunt |
| = 20 mA × 10 |
| = 0.2 V |
| (2) |
Therefore, if the source is configured for constant current, with its voltage referenced to 0.2 V sensed at the the shunt resistor, then the output across (Rload + Rshunt) is 10 V @ 20 mA. You can obtain other current levels the same way. For example, if a drive current of 10 mA is required, the control voltage will be 0.1 V.
Remember, however, that the shunt resistor value must be precise and must be connected on the ground side of the load (see Figure 4) if the analog output is to function properly. Failure to make the proper connections will result in unpredictable behavior from the analog output circuit and may result in damage to the circuit under test.
Quadrants 1 and 3—Battery Charge/Discharge Test
To demonstrate two-quadrant operation, this application tests a battery charge/discharge cycle using a constant current source. The battery will be charged to 9.6 V, then discharged to 1 V.
The charge cycle occurs in quadrant 1, where both the voltage and current are positive. The discharge cycle then occurs in quadrant 3, where the voltage and current are negative. A shunt resistor will be selected, and the voltage drop across it will be maintained to limit the charge and discharge currents to a predetermined safe level (10 mA in this case). The battery’s load resistance is low. Therefore, the calculations that follow assume load resistance to be negligible (< 0.1 ). A 10 shunt resistor will be used.
The analog output voltage is a function of the voltage that is programmed and sensed across the shunt resistor. The acceptable programmed shunt voltage can be determined by using Ohm’s law and the required current:
| Vmaxshunt | = 10 mA × Rshunt |
| = 10 mA × 10 |
| = 0.1 V |
| (3) |
Figure 5. The constant current battery test configuration applies the theory shown in Figure 4, except that the device under test (DUT) is now a battery. By monitoring the voltage drop across RSHUNT, you can adjust the output of the current source automatically to maintain a specific charge or discharge current through the battery, regardless of its charge level.
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You can set the shunt circuit analog output to 0.1 V for the charge cycle and to –0.1 V for the discharge cycle. You can monitor the battery voltage using a PC analog input card and control the test cycle via the user program. For this test, connect the load and shunt resistor to the analog output and the sense lines (see Figure 5).
Quadrants 1 and 2: Source/Sink Test
Performing a battery charge/discharge cycle in constant voltage mode, however, changes the procedure. In this final test, the analog output card will source voltage and sink current, which is a capability that’s rather rare among DA cards.
As in the previous example, the battery first will be charged to a specific voltage, then discharged to a predetermined level. The charge cycle occurs in quadrant 1, where both the voltage and current are positive. The discharge cycle then occurs in quadrant 2, where the voltage is positive and the current is negative. You must select a series resistor to limit current; its value will be based on the
Figure 6. The constant voltage battery test configuration uses a programmable voltage source as the analog output and measures the voltage delivered across the series battery/resistor circuit. Using the sensed voltage, you can adjust the source's output to maintain a specific voltage across the battery and resistor during a charge or discharge cycle.
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voltage and current specifications of the DUT. The load resistance of the battery source is low, so the calculations that follow assume the load resistance to be < 0.1 (see Figure 6).
The analog output voltage will be a function of the voltage programmed and sensed across the series combination of the battery and the resistor. This example assumes that the maximum charge current is 10 mA, the maximum voltage is 9 V, and the minimum voltage is 3 V. You can select a suitable series resistance based on the test requirements:
| Rseries | = Vchange / Imax |
| = (9.0 – 3.0) / 10 mA |
| = 600 |
| (4) |
Now you can set the analog output to 9 V for the charge cycle and 3 V for the discharge cycle. As before, monitor the voltage levels using an analog input card, and control the test cycle from the user program.
ünlike in the constant current example, here the exact current level sourced and sunk by the analog output card varies with the level of charge of the DUT (the battery, in this case). Also, the current flow is reduced as the potential of the DUT approaches that of the programmed analog output. Therefore, it’s critical to select the correct series resistance to limit the maximum current from the analog outputs. Failure to do so can result in damage to the DUT.
Conclusion
Features once unique to benchtop instruments—such as 4-wire sourcing/sensing and multiquadrant operation—are now appearing in DA cards. Four-wire sensing ensures that a DUT is being controlled at the programmed voltage. Multiquadrant operation enables a source to sink or source voltage and current in any combination, providing versatile test options that can simplify test setup and reduce equipment requirements.
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Test System Safety
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Many electrical test systems or instruments can measure or source hazardous voltage levels. It’s also possible, under single-fault conditions (e.g., a programming error or an instrument failure), for a system to output hazardous levels even when it indicates no hazard is present. To protect operators and eliminate potential safety hazards, always follow these safety measures:
- Design test fixtures to prevent operator contact with any hazardous circuit.
- Make sure the device under test is fully enclosed to protect the operator from flying debris. For example, capacitors and semiconductor devices can explode if too high a voltage is applied.
- Double insulate electrical connections that an operator might touch. This ensures that the operator is still protected, even if one insulation layer fails.
- Use high-reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened.
- Use automated handlers so that operators don’t have to manually open guards or access the inside of the test fixture.
- Provide proper training to all users of the system so that they understand potential hazards and know how to protect themselves from injury.
Remember, test system designers, integrators, and installers are responsible for ensuring that effective protection systems are in place for operator and maintenance personnel.
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Jon Semancik is Senior Applications Engineer, Keithley Instruments, Inc., 28775 Aurora Rd., Cleveland, Ohio 44129; 440-498-2694, fax 440-542-8017, jsemancik@keithley.com.
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